The 33 boroughs of Greater London have been designated a Low Emission Zone (LEZ), where vehicles which do not meet certain pollution standards will be discouraged by charging.

When does this begin?

Technically it comes into force on February 4, but the London mayor, Ken Livingstone, has given each operator 28 days to comply from the first time they drive into the zone.

Why is it happening?

London is one of the most polluted cities in Europe for tiny particulates, which exacerbate illnesses including lung disease and asthma, and road vehicles are responsible for two-thirds of this pollution.

Who will have to pay?

The charge will be phased in over several years as more vehicles and tougher standards are incorporated. Initially, only diesel lorries and other vehicles over 12 tonnes manufactured before October 2001 will have to pay. In effect, vehicles built to Euro III standards or better will not have to pay, though the standard will rise in future.

Is there any way of avoiding the charge?

Vehicles can be retrofitted to make engines cleaner, or replace them, and a few are exempt or get full discounts, including farm machinery and cranes. Of 120,000 vehicles over 12 tonnes driving into Greater London last year, one in four would have had to pay the charge, but Transport for London (TfL) expects many operators will take measures to avoid it.

Will everybody be included eventually?

So far the charge extends to buses, coaches and smaller lorries later this year, and to large vans (think Ford Transit) and other vehicles like minibuses in 2010. Emissions standards will rise in 2012.

How much will it cost?

The charge will be £100-200 per day, from midnight to midnight, seven days a week, every day of the year. Penalties for non-payment start at £500-£1,000.

Is this instead of the congestion charge?

No, but Ken Livingstone has announced that, in addition to designating Greater London as an LEZ, he hoped to approve plans for a £25-a-day congestion charge on the highest-polluting vehicles coming into the capital within a month.

How much money is it expected to bring in?

Income is expected to be £5m-11m a year, while the scheme will cost £57m to set up and £10.7m a year to run.

Can businesses afford this?

The Freight Transport Association says small operators will be put out of business because they cannot afford to upgrade or replace vehicles. However all businesses with older vehicles will be equally effected.

What will be the impact on air pollution?

TfL believes the scheme will save 86 hospital admissions each year and by preventing early deaths will save thousands of years of life in the first decade. Improvements in heath have been calculated to be worth £170m-£670m in the first seven years.

Is there no other way to clean up air pollution?

Critics say cars, which make up the vast majority of vehicles in London, should be included in the charge. TfL says pollution from road transport is "largely" caused by bigger vehicles, though it could not specify how much.

Is the M25 included?

Much of the charging zone runs up to but does not include the M25. All other public roads, including small parts of the M1, M4 and the M4 spur to Heathrow airport, are included.

I've never heard of this before ...

There are no other LEZs in the UK, but they are common elsewhere: there are 50 in the rest of Europe.

For maps, information about whether you need to register and how to pay, go visit the TfL website.